Technical Field
Small circuits or circuit elements of feature size of the order of a few micrometers or smaller generally undergo one or more lithographic delineation steps in fabrication. One such category of circuits often fabricated in and on epitaxial silicon is known as large scale integration (LSI). Prevalent manufacture is based on sets of discrete masks patterned by means of ultraviolet light, electron beam or other actinic radiation. An alternative approach, known as "direct processing" is based on primary pattern delineation in masking layers affixed directly to the device or circuit during manufacture.
All such processes involve primary or secondary pattern delineation to eventually permit plating, etching or other modification of underlying active material exclusively in bared regions of apertured masking layers.
Silicon integrated circuitry (SIC) is exemplary of the accelerating development of miniaturized devices. MOS (metal oxide semiconductor) LSI's (large scale integration) are illustrative. State of the art circuitry is represented typically by a chip of dimensions of a small fraction of an inch--perhaps 1/4 inch square--containing as many as 64,000 individual cells each, in turn, containing a number of individual elements. Design rules--smallest line/space dimensions are now about 31/2 micrometers. Such devices are typically fabricated by use of sets of masks with patterns defined in terms of apertured regions in thin chromium layers supported by glass substrates. Mask sets, of perhaps five to fifteen individual masks, serve as primary patterns. Such masks serve to replicate patterns by use of transmitted near UV radiation made incident on suitable actinic material on the device undergoing processing.
Primary pattern generation has gone through a number of stages with sophisticated devices at this time being generated by software control of a monitored travelling beam--generally, an electron beam. Electron sensitive resist material may be negative acting (selectively retained where irradiated) or positive acting. Development of exposed resist is generally wet. The developed pattern is replicated in underlying chromium to result in a so-called "hard" copy mask which is supported on optically polished glass with the entirety serving as the final primary mask.
State of the art replication is showing a trend toward projection printing with replication often 1:1 on the supported photoresist. Once processed, the photoresist then serves to localize any of the various modifications required in fabrication of the device. Examples of such modification include electroplating, ion implantation, etching, etc.
The latter fabrication step--etching--is perhaps the most ubiquitous step in fabrication and, consequently, has represented the largest concentration of developmental effort. Wet etching, generally isotropic by nature, served well and continues to serve where feature dimensions do not approach the usually fractional micrometer layer to be processed. Finer features require directionality, i.e., anisotropic etching, generally with minimal undercutting of resist material. Other considerations, e.g., contamination, ease of removal of spent etchant, etc., give rise to a desire for dry etching, or, more generally, for dry processing.
Dry processing may make use of simple vapor chemical reaction but this, in the usual situation, continues to be isotropic. Anisotropic dry processing generally makes use of directionality imposed by net motion of relevant processing species. In the instance of dry etching, use may be made of plasma environment as in simple plasma etching; directionality may be enhanced by use of a dc bias, as in reactive ion etching or sputter etching, or dependence may be had solely on momentum exchange, as in ion milling.
State of the art devices are controllably fabricated by use of mask technology. Registration ability by present means would seem to permit extension of this technology to design rules of about 2 micrometers or perhaps to about 1 micrometer. Registration precision of masks--typically, several inches in diameter and containing many tens of circuits--is expected to become limiting so that economic considerations may dictate use of a maskless process for small feature size--perhaps for design rules at about 1 micrometer.
Maskless processing, sometimes known as "direct processing", depends upon primary pattern delineation from software directly on a resist layer intimately supported by the device undergoing processing. Direct processing imposes new requirements both on lithographic apparatus and on resist. Apparatus, in addition to high resolution capability, must, in the usual case, be capable of a throughput much more rapid than that expedient for mask production. Design improvisation depending, for example, on shaped, nonGaussian beams, would appear to be a significant part of the solution. Increased throughput gives rise to a requirement for shortened exposure. Exposure depends upon lithographic sensitivity of the resist and brightness of the source. Considerable effort is being directed toward improvement of both.
Primary pattern delineation implies the need for controllably moving and modulating a focused beam of radiation. Most advanced apparatus depends upon electron beam. Materials of sensitivity adequate for mask making have been developed and are now off-the-shelf items. Electron sources, generally tungsten or thorated tungsten, may soon yield to lower work function thermionic emitter materials, such as, lanthanum haxaboride; or may take the form of high density field emitters. The prevailing view is that one or the other approach coupled with newly emerging resist materials will satisfy lithographic requirements certainly through the expected generations of mask fabrication and likely into direct processing.
There has, for some years, been an ongoing interest in the possibility of delineating beams of larger atomic particles. In general, means for generating, accelerating, focusing, and otherwise controlling charged ions have been adequate for production of micron and smaller diameter beams. Development over recent years has been largely directed toward increasing brightness, so that, at this time, means have been described for production of 1 to 1/10 micron diameter beams at 1 amp/cm.sup.2 --representing deposited energy comparable to that of presently used electron beams. Apparatus which has been reported in the literature shows promise of requisite scan rates, focusing, modulation, etc., but it does appear that, for some time, available incident power will continue to be only comparable to electron beam.
It is believed that ion lithography offers potential advantages. A variety of considerations lead to the possibility of improved resolution. A main basis for this expectation is the absence of backscattering of lithographic significance. For the most part, ions are not captured and cannot, therefore, backscatter; while secondary electrons are of significantly lower velocity than the primary electrons used in e-beam lithography. For the most part, it has been assumed that the most promising ion resists would be chosen from among the best electron resists. This continues to be the general consensus and, in fact, the most sensitive e-beam resists are shown to be most sensitive for ion exposure, as well. A problem in ion beam lithography common with that already experienced with electron beam lithography has to do with the need for improved stability, for example, in a variety of processing environments. In e-beam lithography, design to accomplish this end has generally resulted in a loss in sensitivity. So, for example, inclusion of an aryl moiety in the prevalent "COP" (copolymer of glycidyl methacrylate with ethyl acrylate) while increasing processing stability results in the expected decrease in lighographic sensitivity.